Tool for machining a cmc by milling and ultrasonic abrasion

ABSTRACT

The invention relates to a tool for machining hard materials such as metal-matrix or organic-matrix composite materials, comprising a cylindrical sonotrode ( 22 ) connected, along the axis (A) of said cylinder, to an assembly vibrating at a predetermined ultrasonic frequency and rotated about said axis, and at least one nozzle ( 3 ), on the surface to be machined and at the terminal end ( 24 ) of the sonotrode, for ejecting a liquid in which abrasive particles are suspended, said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a drilling head along the axis (A) of the cylinder, characterized in that the circumference ( 23 ) of the sonotrode is covered with particles of a so-called superhard material in order to form a milling head capable of moving in a plane that is substantially perpendicular to said vibratory axis (A).

The invention relates to the field of machining materials and more particularly of machining very hard materials such as ceramic-matrix composites (CMC) or organic-matrix composites (OMC).

Ceramic-matrix composite materials are well known for their difficulty of machining and their abrasive action on the tools used to machine them. They are generally cut or ground by water jet, but the accuracy achieved is low, and few materials, apart from diamond, are capable of machining them properly while having sufficient service life.

Diamond is more commonly used in cutting tools in polycrystalline form, or PCD (Polycrystalline Diamond). PCDs are obtained by sintering diamond particles with a chemico-mechanical binder, such as cobalt, under high pressure and at high temperature. The cobalt binder allows the cohesion of the diamond grains and the combination of the two imparts advantageous cutting properties to the final tool. However, the cobalt matrix is not strong enough and the diamond grains are gradually detached during the machining, making the PCD insufficiently effective for machining ceramics.

Alongside this machining by mechanical contact between the tool and the part to be machined, a method is known for machining composites by ultrasound. It is described in particular in patent EP 0362449 of the Office National d'Etudes et de Recherches Aerospatiales (ONERA). It uses a tool, or sonotrode, connected to an assembly vibrating at an ultrasonic frequency which transmits these vibrations to an abrasive, such as boron carbide. The abrasive is placed in suspension in a liquid which is ejected on the part to be machined, between the end of the sonotrode and the part. The particles have the effect of microhammering the part and eroding it. The tool progressively penetrates into the part, reproducing its own shape. This method presumes that the distance between the sonotrode and the material is properly controlled.

Also known is a French patent application FR 2534512 which describes a sonotrode for polishing a part after machining. This acts similarly to the one described in the patent published by ONERA, that is to say, it is subjected to a longitudinal vibratory motion in the presence of a jet comprising abrasive particles. It is also rotated about itself at relatively low speed (500 to 5000 rpm), to protect the sonotrode from wear that is not circularly uniform. This rotary motion does not participate in the milling.

Regardless of the method considered for machining CMC or OMC parts, it does not allow for a machining capacity that combines high accuracy with satisfactory cutting speed.

It is the object of the present invention to remedy these drawbacks by proposing a method for machining ceramic-matrix or organic-matrix composite materials that operates at relatively high speed and that does not result in excessively rapid wear of the tool employed.

For this purpose, the invention relates to a tool for machining hard materials such as metal-matrix or organic-matrix composite materials, comprising a cylindrical sonotrode connected, along the axis of said cylinder, to an assembly vibrating at a predetermined ultrasonic frequency and rotated about said axis, and at least one nozzle for ejecting, on the surface to be machined and at the terminal end of the sonotrode, a liquid in which abrasive particles are suspended, said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a drilling head along the axis of the cylinder, characterized in that the circumference of the sonotrode is covered with particles of a so-called superhard material in order to form a milling head capable of moving in a plane that is substantially perpendicular to said vibratory axis.

The sonotrode thus acts by a combination of milling actions, both by the terminal end and by its circumference, thereby increasing the speed at which the machining takes place, and/or attacking very hard materials while preserving a reasonable depth of cut.

In a particular embodiment, the so-called superhard material is polycrystalline diamond.

In another embodiment, the so-called superhard material is cubic boron nitride.

Preferably, said abrasive particles are made from boron carbide.

Alternatively, said abrasive particles are made from polycrystalline diamond.

The invention also relates to a method for machining hard materials such as metal-matrix or organic-matrix composite materials, using a tool as described above, the sonotrode vibrating in a direction substantially perpendicular to the surface to be machined, and moving in a plane that is substantially perpendicular to its vibration direction.

Advantageously, the sonotrode is vibrated at a frequency that is substantially equal to 20 kHz.

Preferably, the speed of rotation of said sonotrode is between 10 000 and 40 000 rpm.

Advantageously, the tool operates in successive cuts, the depth of each cut being 0.5 mm or less.

Preferably, the axial feed rate is greater than 500 mm/min.

Even more preferably, the axial feed rate is between 500 and 1000 mm/min.

The invention will be better understood, and other purposes, details, features and advantages thereof will appear more clearly from the detailed explanatory description that follows, of an embodiment of the invention provided as a purely illustrative and nonlimiting example, with reference to the appended schematic drawings.

In these drawings:

FIG. 1 is a schematic view of the operation of an ultrasonic milling cutter according to the prior art;

FIG. 2 is a schematic view of the operation of a rotating and vibrating milling cutter, according to the prior art;

FIG. 3 is a schematic view of the operation of an ultrasonic milling cutter associated with rotation of the tool, according to an embodiment of the invention;

FIGS. 3, 4 and 5 are successive schematic views of the feed motion of a machining operation using a tool according to the invention;

FIG. 7 is a detail view of the machining of a part by the tool according to the invention.

FIG. 1 shows an ultrasonic cutting tool, similar to the one described in patent application EP 0362449, for machining a part 1 positioned opposite a sonotrode 2. The tool transforms an alternating current at a frequency of about 20 kHz, which corresponds in air to the ultrasonic range, into mechanical vibrations having the same frequency that are applied to the sonotrode 2. The sonotrode 2 is subjected to a back-and-forth vibratory motion along a direction A parallel to its axis of symmetry. The sonotrode 2 acts as a cutting tool via very hard abrasive particles, such as boron carbide, which are projected against the material to be machined. Nozzles 3 are positioned for this purpose, next to the sonotrode 2, and send a water jet containing abrasive particles in suspension against the surface to be machined, at the terminal end 4 of the sonotrode. Since the water fully transmits the ultrasonic frequencies, these particles are excited by the vibrations of the sonotrode 2 and vibrate at the same frequency of 20 kHz. They accordingly penetrate into the surface to be machined, causing a deformation that is followed by a removal of material in the form of microchips.

FIG. 2 shows a tool such as a milling cutter or file cutter, for machining by rotation of a tool 12, which is also associated with vibration of the tool. The cutter 12 is covered, at its terminal end forming a drill 14 and on its circumference forming a milling head 13, with an abrasive material, such as polycrystalline diamond, for example. The tool is conventionally rotated by the machine and brought into contact with the material 1 to be machined. In addition to its rotary motion, the tool 12 is vibrated along the direction A parallel to its axis of symmetry, causing periodic attack of the material, like an impact drill.

FIG. 3 shows a cutting tool according to the invention, adapted for machining very hard materials such as CMC or OMC composite materials. As previously, it comprises a tool 22, which acts here as a milling head and sonotrode. For this purpose, on the one hand, it is vibrated along its longitudinal axis at a frequency close to 20 kHz to act as a sonotrode via its terminal end 24 and, on the other hand, it is rotated to act as a milling head via its cylindrical circumference 23. This circumference 23 is covered to a certain height with so-called superhard materials such as polycrystalline diamond or cubic boron nitride, which have a hardness substantially equal to that of diamond. Its terminal end 24 may either be smooth, unlike the tool in FIG. 2, or may be covered with diamond to improve its abrasion resistance. As in the case of FIG. 1, nozzles 3 are placed on either side of the rotating sonotrode 22, and project abrasive particles in suspension in a stream of water directed toward the composite material 1. These particles, having a diameter of a few tens of microns, may be made from boron carbide, silicon carbide or polycrystalline diamond.

With reference to FIGS. 4 to 6, the method used for machining CMC or OMC using a tool as described above, takes place as follows:

The tool 22, covered with polycrystalline diamond, typically has a diameter between 5 and 15 mm and is rotated at a speed between 10 000 and 40 000 rpm. It is also vibrated, along the axis A, at a frequency of kHz by an acoustic unit comprising piezoelectric ceramics, whose mechanical amplitude can be adjusted and varies between 5 and 100 microns. This vibration amplitude remains compatible with the consistant requirement to control the distance between the sonotrode and the material to be machined. A water jet containing boron carbide or diamond particles is injected in front of the rotating sonotrode 22 by means of the nozzles 3.

The tool 22 is first positioned facing the surface to be machined that it attacks (FIG. 4) in the same way that would be done by a sonotrode of the prior art, with an axial advance. Once a predefined depth, called the depth of cut, is reached, the probe continues to vibrate and to be supplied by the nozzles, but begins to move laterally (FIG. 5) to attack the material by its circumference 23 covered with polycrystalline diamond. From this time on, the machining advances (FIG. 6) simultaneously by abrasion of the material located opposite the terminal end 24 of the rotating sonotrode 22 due to the abrasive particles activated by the ultrasound, and by mechanical attack by the lateral surface 23 of the sonotrode. When the material 1 has been machined along its whole length, the operator starts a new cut by returning to the starting point and repeating the operations as shown in FIG. 4.

FIG. 7 shows the way in which the rotating sonotrode 22 penetrates into the material to be machined 1. Its circumference 23 attacks the material over a height h1, while the terminal end 24 excavates the surface of the material over a second height h2. The final height of the cut obtained is equal to the sum of the two heights h1+h2.

A lateral advance into the material is thereby generated, like a file cutter, and in practice, milling is obtained to a depth of up to 0.5 mm per cut, which can be sustained with feed rates between 500 and 1000 mm/min, that is to say, speeds which can be considered as high for machining a CMC or OMC composite material.

The combination of an ultrasonic tool and a water jet mixed with abrasive particles, with a rotating spindle driving a diamond tool, thereby serves to obtain accurate machining and to produce complex shapes with high feed rates, which was unfeasible by the techniques of the prior machines used separately.

Although the invention has been described in relation to a particular embodiment, it is obvious that it comprises all technical equivalents of the means described and combinations thereof, if they fall within the scope of the invention. 

1. A tool for machining hard materials such as metal-matrix or organic-matrix composite materials, comprising a cylindrical sonotrode (22) connected, along the axis (A) of said cylinder, to an assembly vibrating at a predetermined ultrasonic frequency and rotated about said axis, and at least one nozzle (3) for ejecting, on the surface to be machined and at the terminal end (24) of the sonotrode, a liquid in which abrasive particles are suspended, said particles being vibrated at said ultrasonic frequency by the sonotrode so as to form a drilling head along the axis (A) of the cylinder, characterized in that the circumference (23) of the sonotrode is covered with particles of a so-called superhard material in order to form a milling head capable of moving in a plane that is substantially perpendicular to said vibratory axis (A).
 2. The tool as claimed in claim 1, in which the so-called superhard material is polycrystalline diamond.
 3. The tool as claimed in claim 1, in which the so-called superhard material is cubic boron nitride.
 4. The tool as claimed in one of claims 1 to 3, in which said abrasive particles are made from boron carbide.
 5. The tool as claimed in one of claims 1 to 3, in which said abrasive particles are made from polycrystalline diamond.
 6. A method for machining hard materials such as metal-matrix or organic-matrix composite materials, using a tool as claimed in one of claims 1 to 5, the sonotrode (22) vibrating in a direction (A) substantially perpendicular to the surface to be machined, and moving in a plane that is substantially perpendicular to its vibration direction.
 7. The method as claimed in claim 6, in which the sonotrode (22) is vibrated at a frequency that is substantially equal to 20 kHz.
 8. The method as claimed in either of claims 6 and 7, in which the speed of rotation of said sonotrode is between 10 000 and 40 000 rpm.
 9. The method as claimed in one of claims 6 to 8, in which the tool operates in successive cuts, the depth of each cut being 0.5 mm or less.
 10. The method as claimed in one of claims 6 to 9, in which the axial feed rate is greater than 500 mm/min.
 11. The method as claimed in claim 10, in which the axial feed rate is between 500 and 1000 mm/min. 